Effect of side slope of main channels on formation and penetration of scour hole in confluences: Penetration of scour hole in the river confluences

In an open‐channel confluence, deep scour holes and depositional point bars are usually formed due to high bed‐shear stresses and secondary circulations. In the present study, presuming the effectiveness of channel geometry on the flow dynamics at the confluence, some variables including different side slope angles of the main channel (θ = 45°, 60°, 75°, 90°), lateral to downstream discharge ratio (Qᵣ), and downstream densimetric Froude number (Frg₃) were experimentally studied under clear‐water condition for the confluence angle α = 90°. According to the results, the increase of θ led to a greater penetration of scour hole across the main and tributary channels, whereas a little scour development was observed along those channels. Meanwhile, an increase in Qᵣ and Frg₃ caused further scouring, but their effects on the dimensions of scour hole diminished with increase of θ. Thus, with increase of Qᵣ from 0.194 to 0.552, the mean penetration rate of scour hole to all directions for θ = 45°, 60°, 75°, and 90° was obtained 42.8%, 32.4%, 25%, and 20.5%, respectively. In addition, considering the effect of θ, Qᵣ, and Frg₃, some empirical relationships were obtained for estimating the penetration length of scour hole. The derived relationships show that Frg₃ plays more important role on the dimensions of scour hole than θ.